This invention relates generally to water heating apparatus. More particularly, it relates to batch-type water heating apparatus adapted for use in a domestic hot water supply system.
The typical domestic water heater includes a water storage tank and a tank heating unit provided with upper and lower heating devices, generally electric resistance heating elements, respectively associated with upper and lower tank thermostats. The arrangement is such that water in the upper portion of the tank is heated until the upper tank thermostat is satisfied, and then water in the lower portion of the tank is heated until the lower tank thermostat is satisfied. As a result, a limited supply of hot water is replenished in a relatively short period of time, to the extent of the capacity of the upper portion of the tank.
The heat pump has been known for many years. It is generally more costly to purchase initially but more economical to operate than a conventional electric resistance tank heating unit. The heat pump has not been used extensively heretofore because electric power has been relatively inexpensive. As the cost of power increases, there is increasing interest in the use of a heat pump in association with a water heater, especially for providing domestic hot water.
When a heat pump is so used, it is advantageous to make the heat pump responsive to the tank thermostats as an indicator of the demand for hot water. Arrangements of this nature are disclosed to U.S. Pat. No. 4,314,456 issued Feb. 9, 1982, copending U.S. application No. 416,435 filed Sept. 10, 1982, copending U.S. application No. 450,499 filed Dec. 16, 1982 and copending U.S. application No. 497,481 filed May 23, 1983. All are of common assignee herewith, and are incorporated herein by reference.
The U.S. Pat. No. 4,314,456 discloses both a thermostatically operated valve and a by-pass valve. Water leaving the heat pump is maintained at an elevated temperature until the demand in the upper portion of the tank has been satisifed. Thus, much of the time the heat pump is operating at an elevated temperature.
Neither of the two U.S. applications No. 416,435 or 450,499 provides for rapid replenishment of hot water in small batches. Rather, each uses the heat pump to heat an entire tank of water in a single operation.
The U.S. application No. 497,481 provides for replenishment of hot water in portions of the tank. Initially, control means responsive to the upper tank thermostat communicates the upper portion of the tank with the heat pump. Water is circulated from the upper tank portion through the heat pump and back to the upper tank portion until the upper tank thermostat has been satisifed. Subsequently, the control means responds to the lower tank thermostat and communicates the lower tank portion with the heat pump. Water is circulated from the lower tank portion through the heat pump and back to the lower tank portion until the lower tank thermostat has been satisfied. Operation in this manner requires the circulation of water back and forth from the tank to the heat pump.
U.S. Pat. No. 4,363,221 issued Dec. 14, 1982 discloses a water heating system having a heat pump, which system may be operated in either of two modes. In the first mode, an entire tank of water is circulated continuously through the heat pump until the tank thermostat indicates that the desired temperature level has been reached. In the second mode, this circulation is interrupted by a thermostatically controlled valve which is remote from the heat pump. The valve is in effect a throttling valve with a very narrow operating range. Interruption of the circulation introduces operating inefficiencies in the second mode, in that the heat pump operates almost continuously at an elevated temperature. Indeed, it is stated in the patent itself that although rapid recovery of hot water is obtained in the second mode, this is at the expense of thermal efficiency.
There remains a need in the art for batch-type water heating apparatus which will provide for quick recovery; that is, rapid replenishment of the supply of hot water stored in the upper portion of an associated tank. At the same time, the apparatus should be so constructed and arranged that it operates efficiently, is simple, easily constructed and conducive to installation on-site in the field as an adjunct to an existing conventional domestic water heater. Further, it would desirable for the apparatus to be responsive to the tank thermostats as an indicator of the demand for hot water.